An Interaction is the smallest piece of work that is carried out by a Contact Center Agent. Interactions are 'media agnostic' (or 'channel neutral') - all interactions share properties and actions that can be carried out on them regardless of their media type.
A Work Request groups all interactions carried out with a particular Customer. All work that has been carried out with a specific customer can then be reported on by the Contact Center - this information is used to build up a picture of the 'Customer Journey'.
The Customer Interaction Services Client SDK package for Javascript uses the concept of capabilities to indicate the current capability of an Agent to perform an operation.
For example:
Capabilities are set by the Server, and should be taken as an indication that the operation will succeed if attempted. Have a look at the Capabilities Tutorial to see them in action.